The Edge of the Solar System

Scientific American

All good things must come to an end.

For NASA’s Cassini orbiter—its fuel dwindling after 13 years exploring Saturn, along with the planet’s sprawling rings and dozens of icy moons—the end will come Friday at 7:55 A.M. Eastern time. That’s when mission planners project radio communications will be lost with the two-ton, bus-size spacecraft as it plunges into the giant planet’s turbulent atmosphere at more than 122,000 kilometers per hour.

Within seconds the gas streaming around the plummeting probe will reach temperatures hot enough to melt its aluminum chassis, followed by the iridium cladding that shields its plutonium power source. With its radio link to Earth severed, Cassini’s last “transmission” will be the light from this fireball, a modest blaze of glory that astronomers might glimpse from Earth. A minute after its final signal, the sophisticated spacecraft will be reduced to a rapidly dissipating spray of vaporized metal some 200 kilometers beneath Saturn’s swirling cloud tops. But as scientists and members of the public gather in various parts of the world for bittersweet celebrations of the mission’s conclusion, it is clear Cassini’s legacy will endure.

“We are concluding the longest, deepest, most comprehensive scientific exploration of a remote planetary system ever undertaken, a system so alien it might as well have been orbiting another star in another galaxy,” says Carolyn Porco, the planetary scientist who leads Cassini’s imaging team. “And we have been profoundly successful.”

Launched in 1997, Cassini reached Saturn in 2004. With a design that had been mostly finalized in the late 1980s, its suite of 12 instruments had already become antiquated relics back on Earth by the time of its arrival. Even so, it managed to reveal a system more otherworldly than researchers had dreamed of. The highlights are too numerous to list here. Cassini discovered a strange, long-lived hexagonal jet stream sprawled across Saturn’s north pole, and watched as the world and its rings slowly tilted through the seasons in their glacial motion around the sun. Within the rings themselves, Cassini spied tumbling moonlets that sculpted the surrounding dust into towering waves and jagged ridges. But more than the planet or its rings, Cassini studied Saturn’s icy moons—essentially an entire planetary system in miniature, each new world a microcosm of unearthly geology. There was porous, pockmarked Hyperion, which could be mistaken for a piece of spongy pumice; bizarre Iapetus, with color-contrasting hemispheres and a globe-girdling ridge that makes it look like a yin-yang-emblazoned walnut. There was Mimas, with a giant bull’s-eye crater and steel-gray coloring like a space station straight out of Star Wars; and Pan, a beige, pierogi-shaped body that looks good enough to eat. And these are just a sampling of the system’s curiosities.

All told Cassini sent back about 635 gigabytes of data, much of it consisting of gorgeous images. That may seem underwhelming by today’s terabyte-saturated technologies, says Earl Maize, Cassini’s project manager at the NASA Jet Propulsion Laboratory. But getting that amount of data from vintage equipment operating at the far reaches of the solar system “was pretty tall cotton,” Maize says. “We’ve had exceptional performance, and we couldn’t have asked for much more. It has just been a phenomenal ride.”

Dark OceansFlush with so much information about these far-off worlds, it is easy to forget that scarcely more than 40 years ago Saturn and its largest moons were at best blurry images for everyone on Earth. The majestic rings seeming almost cartoonish and somehow unreal in the tremulous views from telescopes. That changed in the 1970s, when first Pioneer 11 and next the Voyager 1 and 2 missions each traveled to Saturn to deliver sharper close-ups. Those missions transformed the ringed planet from a picture to a place, but they were one-and-done flybys—like seeing a foreign city through the windshield of a passing car.

When scientists and engineers began planning Cassini more than three decades ago, they envisioned something altogether different: an orbiter that would be captured by the planet’s gravitational field to loop and twirl among its rings and moons in an exquisite dance of discovery, revisiting each target of interest again and again. But for this dance Cassini needed a partner, something else orbiting Saturn that could give the spacecraft a hefty boost of trajectory-tweaking, fuel-saving momentum on a regular basis. The choice for this long-term relationship was obvious. Saturn’s largest moon, Titan, is bigger than the planet Mercury. With each close pass Titan would give Cassini a gravitational kick akin to about 450 kilograms of propellant (Cassini performed 127 of these encounters during its mission, ultimately netting the equivalent effect of burning more than 50,000 kilograms of fuel).

At the time, Titan also seemed to offer the greatest chance for revolutionary science. Unique among solar system moons, it alone has a dense atmosphere—one so thick with salmon-colored hydrocarbon smog that it had resisted all previous attempts to obtain surface images. Cassini would carry, along with high-powered radar to pierce Titan’s veil of clouds, a European-built lander, Huygens, to touch down on its hidden surface.

Huygens’s landing in early 2005 was supposed to be the mission’s crowning pinnacle, and it did not disappoint. Descending on parachutes through the orange organic haze, the probe recorded the sounds and speeds of gusting winds, and transmitted images of an eerily Earth-like fluvial landscape of valleys, deltas and sinuous channels carved by liquefied hydrocarbons rather than familiar H2O. Huygens came to rest in what looked to be a floodplain strewn with “stones” of water ice polished smooth by flows of liquid methane, and it touched down with a crunch that suggested its slushy landing site was covered in a frozen glaze—a bit like crème brûlée. Within hours, however, its chemical batteries succumbed to the moon’s cryogenic chill and the lander fell silent. The feat remains by far the most distant interplanetary landfall ever achieved. It gave crucial context to Cassini’s work over the years as the orbiter studied Titan’s weather, seasons and even its transient hydrocarbon lakes and seas.

Shortly after Huygens drifted down to Titan and into the history books, however, Cassini found something that permanently pushed that accomplishment out of the limelight. Around the south pole of Enceladus—a 500-kilometer-wide runt of a moon many expected to be rather inert and uninteresting—the orbiter saw tantalizing signs of activity—plumes of water vapor venting into space from fissures in the icy surface. Wetness and warmth somehow lurked below, offering the possibility for microbial life to arise and thrive—and the plumes were a window into that netherworld. On subsequent passes Cassini dived directly over and through them to sample their contents. Those investigations showed not only that there was an entire ocean of briny water beneath the ice, but also that hydrothermal activity taking place on and under this alien sea could provide abundant energy and nutrients for ecosystems. Scientists are still struggling to understand exactly how such a small moon can generate sufficient heat to maintain a subsurface ocean so far from the sun. Later in the mission Cassini found tentative evidence for an aqueous ocean beneath Titan’s water-ice crust, too.

“Enceladus may have all of the ingredients needed for life as we know it to currently exist right now, at this very second,” says Curt Niebur, Cassini’s program scientist at NASA Headquarters. “These two new worlds, Titan and Enceladus, that were so completely revealed to us by Cassini, have changed the idea that ‘ocean worlds’ like Earth and [Jupiter’s moon] Europa are rare in the universe. … [The ocean of] Enceladus has no business existing—and yet there it is, practically screaming at us: ‘Look at me, I completely invalidate all your assumptions about the solar system!’” Spurred chiefly by Cassini’s results (and at the surprisingly bipartisan behest of Congress), in 2016 NASA began planning a new “Ocean Worlds” exploration program to develop missions that will peer deeper into the seas of the outer solar system. Its next phase will be the Europa Clipper mission, a Jupiter orbiter launching in the 2020s to study the watery depths beneath the Jovian moon’s icy crust. Perhaps it will taste the tang of extraterrestrial sea salt via Enceladus-style plumes thought to erupt there, too. Other factions within planetary-science circles would prefer to send another mission to Titan—maybe even an airborne drone, a sailboat or a submarine—or to reach for farther frontiers by sending orbiters to Uranus and Neptune, the last two planets that remain scarcely explored.

But for Porco and most of the rest of the Cassini team, Enceladus remains the favored port of call—and the mission’s most promising progeny. “Enceladus has all the formal hallmarks of an extraterrestrial ‘habitable zone,’” she says. “No other object in the solar system can claim that. It’s the most accessible and promising place in the solar system to search for extraterrestrial life. We must go back.”

The Last Picture ShowThe orbiter’s dazzling explorations of Titan, Enceladus and other unexpected riches at Saturn convinced NASA to repeatedly extend its mission—first to 2010 and finally to September 2017. But now, with its fuel tanks almost empty from all its wandering, Cassini has fallen victim to its own success; to avoid the spacecraft spinning out of control—striking and potentially contaminating Enceladus or Titan with any lingering Earth-born microbes—mission planners are deliberately crashing Cassini into Saturn, turning it into a makeshift atmospheric probe. As it plunges into Saturn’s clouds it will operate eight of its instruments and use its thrusters to keep its high-gain antenna pointed toward Earth as long as possible, transmitting as much information as it can about the atmosphere’s composition until it is overwhelmed and tumbles into oblivion. Some think that data could help scientists refine their understanding of Saturn’s origins and early evolution.

This “Grand Finale” properly began April 24 when Cassini initiated a new series of 22 Titan flybys. With each pass, the moon’s gravity tweaked the spacecraft’s trajectory to send it whizzing between Saturn’s innermost ring and the top of the planet’s atmosphere—a region never explored before. During these close passes Cassini sampled gases flowing out to space from Saturn as well as dust raining down from the rings. The craft also sought fresh insights into some of the world’s longest-standing enigmas such as the length of the planet’s day and the ages of its iconic rings. It failed to pin down the day length due to lingering difficulties in measuring the rotation rate of the planet’s innards beneath the thick outer envelope of gas.

But its progress on the ages of the rings is more promising: By precisely measuring Saturn’s mass during its finale, the probe is also refining mass estimates for the rings. Many scientists believe a relatively massive ring system would indicate an age of billions of years, suggesting the rings formed with or shortly after Saturn itself. Lighter-weight rings would likely be relatively youthful, at less than a billion years. “Our very first estimates are for rings less massive than we originally thought, which would indeed point to a very young age,” says Linda Spilker, Cassini’s project scientist at JPL. “We’re headed in the direction of rings perhaps 100 million years old or so, which is quite young compared to the age of our solar system” (about 4.6 billion years). Although still quite tentative, this result from Cassini—one of its last—would also be among its most sensational. To form such young rings, Saturn would have needed to tear apart one or more large comets or entire moons in the not-too-distant past—the diversity of the rings, in fact, may be due to multiple objects being consumed over time.

All this adds to the still-emerging picture of our solar system as a dynamic place of continual upheaval, not a static and placid domain as imagined by many scientists in the past. It also hints at yet another aspect of what may be our current privileged place in our solar system’s lifetime: Take most any given slice of Earth history—the dominion of the dinosaurs, for instance—and any creature would have dwelled beneath a sky bereft of Saturn’s ringed splendor. Our current view seems to be anomalous—just as special, at least, as being alive during the fleeting moment in which humanity’s robotic emissaries provide us an expansive new view of our place in the panoply of worlds circling the sun.

In the hours before Cassini becomes a miniscule spark spiraling into Saturn’s massive atmosphere, one of its final tasks will be capturing and sending back images of its most cherished targets: Titan and its methano-logical weather; Enceladus and its watery plumage setting behind Saturn’s northern rim; a moonlet nicknamed “Peggy” being born at the edge of a ring; and a family portrait of the planet and its rings. The spacecraft team calls this “the last picture show.”

“These final images are sort of like taking a last look around your house or apartment just before you move out,” Spilker says. “You walk around the downstairs. As you go upstairs, you run your fingers along the bannister. You look at your old room and memories across the years come flooding back. And in the same way Cassini is taking a last look around the Saturn system, Cassini’s home for the last 13 years.” Like many of her colleagues, Spilker first began working on Cassini in the 1980s, some 30 years ago—about the same time as it takes Saturn to make one full orbit around the sun, and time enough to get family-close to colleagues, to raise children, to watch them grow. Given the fuel, Spilker says another decade spent at Saturn with Cassini would seem about right. But now the time is passed and the fuel is gone, the team has grown older and their children have grown so tall. The next frontier beckons, bright but uncertain. “It’s time,” Spilker says. “Time to take our knowledge and information and move that out into future missions, to the outer planets—and beyond.”

Lee Billings is an associate editor for Scientific American. He covers space and physics.